This invention relates to the formulation of colloidal slurry used to polish NiP plated substrates. This formulation significantly increases the material removal rate during polishing, reduces polish defects, and improves the polished surface finish.
A metallic magnetic thin film disk used in a computer disk drive typically comprises an aluminum substrate plated with NiP, an underlayer sputtered onto the plated NiP, a magnetic Co alloy sputtered onto the underlayer, a carbon protective overcoat sputtered onto the magnetic layer and a lubricant layer deposited on the carbon.
Before depositing the underlayer, the plated NiP is polished to remove surface defects and to lower surface roughness which strongly affects the flying height of a recording head over the disk. A increasing recording density in computer disk drives. At present, the lowest surface roughness Ra obtained using commercially available slurries for NiP plated substrate polishing is about 0.5 nm. ("Ra" is a well-known measure of surface roughness.) However, polishing defects become severe problems as smaller abrasive particles are used in the slurry to reduce the polished surface roughness. Two kinds of polish defects, micro-scratches and polish pits, are apt to form on the NiP polished surface. In general, micro-scratches are thought to be caused by large agglomerated particles. Polish pits are formed either by chemical attacking or other unknown causes.
Current commercially available slurries used for polishing NiP plated substrates typically comprise two components: alumina abrasive particles and an acidic etchant. The abrasive particle size ranges from 0.1 .mu.m to about 1 .mu.m. The slurry pH ranges from 2 to 6 for various polish process applications. Polishing with these slurries is based on micro-machining, wherein the abrasive has an angular shape and grinds the polished surface. The acidic etchant helps increase the efficiency of the micro-machining and improves the polished surface finish by chemical etching. Although a smoother polished surface can be obtained by using smaller abrasive particles, it is still not possible to make scratch-free polished surfaces. Further, these conventional slurries are apt to cause polish pits. This becomes worse with decreasing abrasive particle size to lower surface roughness. Also, smaller abrasive particles create more numerous polishing pits.
In order to eliminate the above-mentioned problems of conventional slurries, colloidal silica has been considered for polishing NiP plated substrates. Colloidal silica has long been successfully used for polishing various materials, such as silicon, gallium arsenide, indium phosphide and titanium, to form a super-smooth and scratch-free surface finish. Colloidal silica slurries used for chemical-mechanical polishing (CMP) typically include aqueous colloidal silica with an etchant (oxidizer) as a polishing promoter. Various kinds of chemicals are used in colloidal silica slurries for different polishing applications to achieve either a high material removal rate or better polished surface finishes with fewer polish defects. Alkaline chemicals, for instance, are used as etchants in colloidal silica slurries to reduce surface roughness in semiconductor wafer rough polish processes as described in U.S. Pat. No. 5,571,373 issued Nov. 5, 1996 to Krishna et al., incorporated herein by reference. Persulphate, as described in U.S. Pat. No. 5,575,837 issued Nov. 19, 1996 to Kodama et al., is used as an etchant in a colloidal silica slurry for mirror-finishing metal surfaces.
Unfortunately, several problems are encountered when attempting to use these commercially available colloidal silica slurries to polish NiP plated substrates. For example, existing commercial colloidal silica slurries, either with an alkaline etchant or an acidic etchant, exhibit a very low NiP removal rate. Further, these slurries also cause polish pits, which are caused by chemical attacking, and micro-scratches. There is presently no adequate colloidal silica formulation for polishing NiP.
To make a colloidal silica slurry applicable to NiP plated substrate polishing, a new formulation is desired to increase slurry's NiP removal rate and to decrease polish defects. It is known in the art that adding an oxidizer or changing chemistry can increase the material removal rate or remove polish defects. However, the extent to which one can add an oxidizer or change the slurry chemistry is bounded by colloidal chemistry as described by I. Ali et al. in "Charged Particle in Process Liquids", published in Semiconductor Intl., in 1990. The colloidal suspension may be broken or the aqueous colloidal abrasive can jell due to the pH value change caused by oxidizer addition or other chemistry change. On-line hydrogen peroxide addition was used by the inventor to improve the polish performance of colloidal silica slurry. (By "on-line hydrogen peroxide addition," I mean that hydrogen peroxide was added to the slurry shortly before use.) A better polished surface finish was obtained by adding hydrogen peroxide to the slurry. However, the NiP removal rate was not significantly increased by adding hydrogen peroxide. Further, hydrogen peroxide cannot remove polish scratches.
Because of the above-mentioned low NiP removal rate and polish defect problems, it would be desirable to make a new formulation which would increase the NiP removal rate of the colloidal slurry and reduce the number of polish defects, and simultaneously provide a good polished surface finish.